In recent years, thin film transistors (TFTs) including a channel layer of a polycrystalline transparent conductive ZnO-based oxide thin film have actively been developed (Japanese Patent Application Laid-Open No. 2002-076356).
Japanese Patent Application Laid-Open No. 2002-076356 states that because the thin film can be formed at low temperature and is transparent to visible light, a flexible transparent TFT can be formed on a substrate such as a plastic plate or a film.
However, a ZnO-based compound cannot form a stable amorphous phase at room temperature, but forms a polycrystalline phase. It is therefore difficult to increase electron mobility because of scattering at grain boundaries. Furthermore, the shape and the interconnection of polycrystalline grains vary widely with the film-forming method. This also produces variations in characteristics of TFT devices.
To solve the above-mentioned problems, recently, a thin film transistor containing an In—Ga—Zn—O amorphous oxide has been reported (K. Nomura et. al., Nature 432, 488 (2004)). This transistor can be formed on a plastic substrate or a glass substrate at room temperature. In addition, the transistor has a field-effect mobility approximately in the range of 6 to 9 cm2/Vs and of a normally-off type. Furthermore, the transistor is transparent to visible light. Further, by irradiating an In—Ga—Zn—O amorphous oxide active layer with an energy beam such as an electron beam or a particle beam, the conductivity can be changed (US 2007/0054507 A1).
Further, it has been reported that when annealing is performed in an atmosphere containing moisture, which is known as steam oxidation, strong oxidative power of steam can improve dielectric strength of a gate insulating layer composed of a silicon oxide film and can reform an interface between a semiconductor and an oxide insulating layer (Japanese Patent No. 3225268).
In addition, Japanese Patent Application Laid-Open No. 2007-311404 discloses a production process including a step in which after an oxide semiconductor is formed, heat treatment is carried out in an oxidative gas atmosphere in order to obtain an oxide semiconductor transistor stable in long-term operation. It is also disclosed that oxygen radicals, oxygen, steam or ozone is used as the oxidative gas.
TFTs containing amorphous oxides, including amorphous In—Ga—Zn—O, were studied, and it was found that the transistor characteristics of the TFTs vary in some cases.
The variations in transistor characteristics can cause variations in operation of organic light-emitting diodes (LEDs) and liquid crystals driven by the transistors, for example, in a pixel circuit of a display. The variations may be caused by parasitic resistance generated between a source electrode and a channel and between a drain electrode and a channel.
The present invention has been made in view of the above problems, and therefore an object of the present invention is to provide a method for manufacturing a field-effect transistor which can reduce the above-mentioned parasitic resistance.